The present invention relates generally to an improved tank process for plating aluminum piece parts such, for example, as porous aluminum castings and, to blister-free plated aluminum substrates produced thereby; and, more particularly, to an improved process, and improved products produced thereby, for plating aluminum substrates with an electrically conductive surface--such, for example, as tin plate--wherein the plating deposit is securely bonded to the aluminum substrate, even where the substrate comprises a porous aluminum casting, with excellent adhesion properties throughout substantially the entire surface area of the substrate to be plated in a substantially blister-free state and, wherein the plating is not subject to separation and/or flaking and possesses improved corrosion-resistance relative to the bare substrate. In its more detailed aspects, the present invention relates to an improved process for forming essentially blister-free plated aluminum substrates, including plated porous aluminum castings, at relatively high production rates when compared with conventional plating techniques; yet, wherein the process yield rates of acceptable quality product are relatively high--viz., yield rates approaching 100%--irrespective of whether the aluminum casting microstructure is of premium quality or inferior quality.
Aluminum castings--including both sand and investment castings--are, subject to the limitations noted below, highly advantageous and desirable for usage in a wide range of product and/or industrial applications, not only because of their strength and light-weight characteristics but, also, because processes permit economical manufacture of even complex structural configurations. However, severe constraints have theretofore been placed on the extent of usage of aluminum castings as a direct result of their poor corrosion resistance characteristics, as well as the fact that such castings tend to spontaneously form non-conductive surface oxides which diminish the electrical conductivity of the casting. Moreover, in those instances where the plated aluminum product is to be used for electrical hardware, it is necessary that the plated substrate exhibit low electrical contact resistance between the substrate and the outermost plating deposited thereon. Sometimes special annodizing processes are used to generate a relatively thick and porous oxide film on the substrate to provide mechanical keying of the subsequent plating deposits. Although adhesion may be good, the oxide layer is non-conductive between the substrate and the outermost plating deposited thereon. Consequently, such special annodizing processes have proven unsatisfactory when attempting to plate aluminum substrates for usage in electrical hardware applications, principally because of their failure to meet the critical requirement of a low resistance interface between the substrate and the outer conductive metal plating deposited thereon.
For these reasons, there has been, and continues to be, an urgent demand for an effective plating process for porous aluminum castings. However, prior to the advent of the present invention, there has been no known, effective and reliable process for consistently depositing other metals on the surface of a porous aluminum casting with good adhesion properties, in a substantially blister-free state and, in those instances where the plated product is to be used for electrical hardware, with a low resistance interface between the substrate and the outer conductive metal layer deposited thereon.
The problems with plating aluminum and alloys thereof have long been recognized by persons skilled in the electroplating industry. For example, as recognized by S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and Its Alloys, 4th Edition, Vol. 2, at page 871 (Published by Robert Draper, Ltd.), aluminum and aluminum alloys are subject to spontaneous formation of oxide coatings which tend to hinder adhesion of subsequent deposits. Because of the amphoteric nature of the oxide produced, the reactions likely to occur in the process are complicated, thereby reducing process reliability. Moreover, the electropositive nature of aluminum and its alloys serves to promote formation of low adhesion deposits. And, since the coefficient of expansion of aluminum differs substantially from those metals which are commonly deposited on it during a plating process, the plated casting has heretofore been advantageously used only in environmental conditions wherein temperature changes are of minimal magnitude--otherwise, differential expansion between the aluminum substrate and the plating deposited thereon can cause sufficient strain to rupture the bond between the substrate and the plating.
In addition to the foregoing problems which are inherent when attempting to plate aluminum and aluminum alloys, those skilled in the art have experienced many other significant problems when attempting to plate porous aluminum castings. See, e.g., F. L. Mickelson, "Problems in Finishing Aluminum Castings", Plating, November, 1966, pages 1319-1322. Thus, Mickelson has recognized that the grain structure of such aluminum castings tends to permit formation of non-conductive precipitates in the grain of the casting substrate when the casting melt is cooled; such non-conductive precipitates commonly including Mg.sub.2 Si, Al.sub.3 Mg.sub.2 and CuAl.sub.2, all of which tend to cause blistering of plated areas and the presence of undesired random unplated areas. Mickelson has further pointed out that surface contaminations such, for example, as mold releases, imbedded iron, magnesium or sand, and thick hard oxides, can ultimately lead to blistering and/or resulting random unplated surface areas. Moreover, the porosity of aluminum castings resulting from the presence of highly soluble hydrogen in the molten aluminum (see, also, E. Player, Symposium on Aluminum Alloy Castings, London, The Aluminum Development Association, 1953, page 110) also tends to promote surface contamination since solutions used in chemical processing (other than rinse water) are often trapped within surface porosities and are, therefore, carried into the next process step as surface contaminants. Thus, if the aluminum melt is not properly degassed at the foundry, the casting tends to be extremely porous and leads to the presence of undesired surface contaminants in the holes at the surface of the casting. It is further to be noted that any unplated surface porosity is also a target of high pH (i.e., a pH greater than 10) solution attack, thus producing both blistering and a vulnerable area for corrosion when the plated casting is in service.
Another significant problem area recognized by Mickelson is the need to remove smut which often leads to blistering of the final deposit, apparently as a direct result of adhesion failure between the porous cast aluminum substrate and the protective coating plated thereon which is commonly either a zincate or a stannate coating. Mickelson suggests nitric-hydrofluoric acid etching to solve the smut problem; but, recognizes that the process is very difficult to control, heats us rapidly, gives off dangerous nitric oxide fumes, and tends to roughly etch the casting. A further problem area delineated by Mickelson resides in the fact that the silicon oxide on the surface of aluminum castings produces a non-wettable (hydrophobic) surface which does not accept plating deposits. Consequently, it is necessary to hydrate or partially remove such oxides by treatment with HF in the presence of water to produce a wettable (hydrophilic) surface. Silicon particle size, which is controlled by the cooling rate of the aluminum melt, is a significant factor in obtaining consistently good plating adhesion. Thus, large high surface area particles produce the largest non-wettable areas and, consequently, the greatest blistering problem--a problem which is more significant when dealing with investment castings than when dealing with quality sand castings. Yet another problem that Mickelson has recognized leads to blistering and to the production of randomly located uplated areas is the fact that microcracks in an aluminum casting tend to absorb oil during the machining process and, subsequent treatment of the workpiece with hot cleaning and/or etching solutions tends to leach the oil from the microcracks, thereby promoting blistering.
Mickelson has proposed a process for plating aluminum castings with chromium which employs a hot alkaline etching step followed by a nitric-hydrofluoric acid desmutting step--steps which he suggests might be eliminated when dealing with "castings of good surface quality" (see, Mickelson, supra, at page 1322). However, it has been found in practice that aluminum castings plated in accordance with conventional processes, such as the process described by Mickelson, are subject to blistering, flaking and separation between the plating and substrate even when the casting has good surface qualities and such etching/desmutting steps are carried out.
The prior art is replete with proposed processes for plating aluminum, aluminum alloys, other metals and/or metallic alloys and, even porous aluminum castings. Such disclosures may, for example, be found in: Peters et al U.S. Pat. No. 3,466,156 and Stone et al. U.S. Pat. No. 3,738,818 (processes for plating aluminum and/or aluminum alloys wherein an electroless nickel plate is deposited on a double zincate coating on the substrate); Simon U.S. Pat. No. 3,180,715 (a process for cobalt plating of 6061-T6 wrought aluminum utilizing an ALUMON.RTM.--a registered trademark of Enthone Incorporated, West Haven, Conn.--double zincate process); Colonel U.S. Pat. No. 3,281,266 (a process for electroless nickel plating over a double zincate protective coating with an intermediate 1-2 minute acid soak); Dunlop, Jr. et al. U.S. Pat. No. 3,202,529 (a process for nickel-cobalt plating of pure aluminum and aluminum alloys employing a zincate coating); Wright et al. U.S. Pat. No. 3,666,529 (a process for electroless nickel plating of 1100, 2024, 3003, 5052, 6061 and 7075 wrought aluminum); Dean U.S. Pat. No. 3,681,019 (a process for zinc plating of aluminum and/or aluminum alloys); Bernstein U.S. Pat. No. 4,115,604 (a process for plating wrought sheet aluminum); a literature review authored by Dr. D. S. Lashmore entitled "Immersion Deposit Pretreatments for Electroplating on Aluminum", Plating & Surface Finishing, April, 1978, pages 44-47 (a review of numerous immersion pretreatment used with aluminum substrates); Hoover et al. U.S. Pat. No. 2,407,881 (a process for depositing a zinc coating on a steel substrate); and, Jones et al. U.S. Pat. No. 3,498,823 (a process for electroless nickel and tin plating on copper substrates).
While none of the foregoing prior art references pertain to, or disclose, processes purported to be useful in plating of porous aluminum castings, the aforesaid Mickelson article, supra, does disclose the problems encountered when attempting to plate such porous aluminum castings and a proposed process for plating such castings. Similarly, Coll-Palagos U.S. Pat. No. 3,726,771 purports to disclose a process for chemically plating nickel on " . . . any type of aluminum and its alloys . . . " (Col. 3, line 34) and wherein it is stated that the aluminum can be " . . . cast, wrought, extruded . . . " (see, e.g., Col. 3, lines 33-43). The patentee goes onto describe an " . . . alternate procedure . . . " which should be used " . . . with aluminum having a high degee of silicate content . . . " (Col. 3, line 44 et seq). However, the specific processes disclosed in the Mickelson article and in the Coll-Palagos patent are known to produce poor results when attempting to plate porous aluminum castings.